The Dark Side of Polar Day - The influence of coastal run-off on Arctic kelp communities

Ecosystem engineering kelp forests are subject to many rapid environmental changes in the Arctic. Since the 1980s, the rate of sea surface temperature rise is far beyond the global average. Consequently, glacial and permafrost melt are accelerating, leading to extensive run-off plumes covering fjords. Run-off plumes alter many water column parameters: e.g., high concentrations of suspended particles are changing the photosynthetically active radiation (PAR); terrestrial and lithogenic material alter the macro- and micronutrient concentrations. To be able to maintain a stable population, kelps have to acclimatise to the changing environment. The overarching aim of my doctoral thesis is to assess what defines the boundaries of the realised niche of Arctic kelp forests. In two in-situ monitoring studies, I found high intraspecific variability in the biochemical composition of different Arctic kelp populations, being conditioned by their local environment along the west coast of Svalbard. Hence, performance curves must not be considered static and experimental results have to be extrapolated with care. Investigating the in-situ effect of run-off on kelp holobiont functioning in a high spatial resolution, I additionally found run-off to change the content of (harmful) elements in kelps. This led to changes of the ecosystem services of kelp forests, such as their nutritious value for grazers, or the element cycling of the ecosystem. In three experimental studies, I found that the effect of temperature on kelps is highly interactive with the prevailing PAR availabilities. High-PAR availability caused drastic physiological stress, especially when interacting with cold temperatures, which currently restricts L. hyperborea to expand to higher latitudes. Reduced irradiance resulted in a shift of the kelps’ realised niche and a local loss of habitat for cold-temperate kelps in the Arctic. Concluding, climate change induced Arctic run-off, and especially varying PAR availability, has drastic consequences on the performance of kelps, with the potential to limit Arctic kelp distribution. A shift of the kelps’ distribution has cascading ecological and economic consequences.